Vehicle engines emit nitrogen oxides (NOx), including NO2, NO, and N2O. Generally, diesel engines emit higher levels of NOx than gasoline engines, because generally, diesel engines operate at higher combustion temperatures and with a higher air/fuel intake ratio. Government emission regulations mandate that diesel vehicles meet NOx emission level output requirements. However, reducing the combustion temperatures and reducing the air/fuel intake ratio can reduce the diesel engine's efficiency and can increase the amount of hydrocarbons, carbon monoxide, and soot in the exhaust gas.
To meet emissions regulations, diesel vehicles can incorporate an exhaust treatment system that reduces NOx. In the application, the exhaust treatment system is equipped to use a urea delivery system and a selective catalytic reduction (SCR) catalyst to reduce NOx emissions. However, the SCR catalyst can have degraded operational performance when the SCR catalyst approaches an end of its operational life.
Therefore, there is a need for a system for monitoring operation of an exhaust gas treatment system using an SCR catalyst to detect degraded operational performance of the exhaust gas treatment system.